1. Field of the Invention
This invention relates to an improved process for the melt extrusion of difficulty-melt-processible polymers.
2. Background
In melt extrusion of polymer resins there are often flow regimes, determined by the rheological properties of the particular resin, where anomalous flow behavior occurs leading to surface imperfections on the extrudate surfaces. Such imperfections, commonly called melt fracture, appear in different forms. The so-called "sharkskin" fracture occurs at lower shear rates and appears as a general, finely-structured and uniform roughness. It may be due to a slip in the die land area and exit region, although there is still controversy over the exact cause (see Kalika and Denn in Journal of Rheology, Vol 31, pp 815-834, (1987)). In a blown-film extrusion, sharkskin fracture may appear as an undesirable herringbone pattern, reducing clarity and giving a dull surface. In practice this may occur at uneconomically low extrusion rates. At higher shear rates flow often becomes unstable and a non-uniform stick-slip melt fracture results, wherein alternating bands of glossy surface and sharkskin fracture appear. This behavior is especially undesirable in wire coating and in tube and pipe extrusions, as well as in blown film processes. Other recognized problems that create difficulties in extrusion include poor pumping that leads to fluctuations in pressure and results in uneven output, accumulation of degraded polymer and polymer additives at the die exit orifice, and the like.
In an effort to improve the extrusion behavior of polymer resins through metal dies it is known to coat the die surfaces that contact the flowing polymer melt with a slip agent, such as tetrafluoroethylene polymers and copolymers, as in Japanese Application Publication Kokai 55-82784 (Mitsui Petrochem. Ind., KK), but bonding to the metal is poor, and over a period of time in use the slip layer is depleted and melt fracture resumes.
In other practices, as for example in the extrusion of certain hydrocarbon polymers and copolymers, it is known to employ small amounts of fluorocarbon polymers, blended with the extrusion resin, as a continuously replenishing slip agent. Thus Blatz, in U.S. Pat. No. 3,125,547, discloses hydrocarbon polymer compositions having improved extrusion behavior that contain small amounts of fluorocarbon polymers that are above their glass transition temperature if amorphous or above their crystalline melting point, if crystalline (e.g. molten) at the process temperatures. Under these conditions the flow rate above which melt fracture occurs is increased, and required extrusion pressures for a given extrusion rate are diminished. Takeshi and Inui in Japanese Examined Application Kokoku 70-30574 disclose continuous extrusion molding of polyethylene compositions containing small amounts of tetrafluoroethylene polymer (crystalline at process temperatures). U.S. Pat. No. 4,904,735 discloses, as process aids, synergistic combinations of fluoropolymers that are molten at process temperatures, such as fluoroelastomers, and those that are not molten at process temperatures, such as crystalline tetrafluoroethylene homopolymers and copolymers.
In the practice of these teachings, wherein a fluoropolymer process aid is added to the extruding resin, the fluoropolymers contact metal surfaces of the extruder and die parts and preferentially collect there, thus providing a low surface energy slip layer between the extruding resin melt and the metal surfaces. However, it is a deficiency of these systems that the fluoropolymers are only weakly adhered to the polymer-contacting parts of the extruder die. As a result, when the extruding resin contains mineral fillers such as are commonly added to serve as antiblocking agents or pigments, the beneficial slip layer may be removed,;for example, by abrasion, as rapidly as it is deposited, and all or part of the beneficial effect of the fluoropolymer additive is lost.
It is an object of this invention to provide a process for the extrusion of difficultly-melt-processible resins containing fluoropolymer process aids through a die wherein at least the polymer-contacting exit region of the die has been specially treated so as to improve the adhesion of the fluoropolymer process aid to the metal surfaces, such that the effectiveness and efficiency of the process aids are improved and the detrimental effect of mineral fillers is greatly minimized.